For example, a hot-rolled steel sheet used in cars, industrial machines and the like is generally manufactured through a rough-rolling process and a finish-rolling process. FIG. 21 is a view schematically illustrating a method for manufacturing a hot-rolled steel sheet of the related art. In the process for manufacturing a hot-rolled steel sheet, first, a slab S obtained by continuously casting molten steel having an adjusted predetermined composition is rolled using a roughing mill 201, and then, furthermore, hot-rolled using a finishing mill 203 constituted by a plurality of rolling stands 202a to 202d, thereby forming a hot-rolled steel sheet H having a predetermined thickness. In addition, the hot-rolled steel sheet H is cooled using cooling water supplied from a cooling apparatus 211, and then coiled into a coil shape using a coiling apparatus 212.
The cooling apparatus 211 is generally a facility for carrying out so-called laminar cooling on the hot-rolled steel sheet H transported from the finishing mill 203. The cooling apparatus 211 sprays the cooling water on the top surface of the hot-rolled steel sheet H moving on a run-out table from the top in the vertical direction in a water jet form through a cooling nozzle, and, simultaneously, sprays the cooling water on the bottom surface of the hot-rolled steel sheet H through a pipe laminar in a water jet form, thereby cooling the hot-rolled steel sheet H.
In addition, for example, Patent Document 1 discloses a technique of the related art which reduces the difference in surface temperature between the top and bottom surfaces of a thick steel sheet, thereby preventing the shape of the steel sheet from becoming defective. According to the technique disclosed in Patent Document 1, the water volume ratio of cooling water supplied to the top surface and the bottom surface of the steel sheet is adjusted based on the difference in surface temperature obtained by simultaneously measuring the surface temperatures of the top surface and the bottom surface of the steel sheet using a thermometer when the steel sheet is cooled using a cooling apparatus.
In addition, for example, Patent Document 2 discloses a technique that cools a rolled material between two adjacent stands in a finishing mill using a sprayer, thereby beginning and completing the γ-α transformation of the rolled material so as to prevent sheet-threading performance between the stands from deteriorating.
In addition, for example, Patent Document 3 discloses a technique that measures the steepness at the tip of a steel sheet using a steepness meter installed on the exit side of a mill, and prevents the steel sheet from being perforated by adjusting the flow rate of cooling water to be different in the width direction based on the measured steepness.
Furthermore, for example, Patent Document 4 discloses a technique that aims to solve a wave-shaped sheet thickness distribution in the sheet width direction of a hot-rolled steel sheet and to make uniform the sheet thickness in the sheet width direction, and controls the difference between the maximum heat transmissibility and the minimum heat transmissibility in the sheet width direction of the hot-rolled steel sheet to be in a range of predetermined values.
Here, there are cases in which the hot-rolled steel sheet H manufactured using the manufacturing method illustrated in FIG. 21 forms a wave shape in the rolling direction (the arrow direction in FIG. 22) on transportation rolls 220 in the run-out table (hereinafter sometimes referred to as “ROT”) in the cooling apparatus 211 as illustrated in FIG. 22. In this case, the top surface and the bottom surface of the hot-rolled steel sheet H are not uniformly cooled. That is, there was a problem in that, due to cooling deviation caused by the wave shape of the hot-rolled steel sheet H, it became impossible to uniformly cool the steel sheet in the rolling direction.
Therefore, for example, Patent Document 5 discloses a technique that, in a steel sheet formed into a wave shape in the rolling direction, makes uniform the cooling capabilities of top portion cooling and bottom portion cooling so as to minimize the influence of the distance between soaked water on the top portion of the steel sheet and a table roller at the bottom portion in order to uniformly cool the steel sheet.